1. Field of the Invention
This invention relates to the manufacture of panels of reinforced plastic foam, and particularly to a method for imparting a uniform thickness to the panels without diminishing the requisite properties of the panels.
2. Description of the Prior Art
Methods for the continuous production of a rigid insulating polymeric foam board comprising a foam layer sandwiched between facing materials are well known. Such boards are typically used in the construction of walls, ceilings, roofs and the like. In the production, a foam-forming mixture is customarily delivered between two continuous webs of sheet material which are transported through various processing stations.
In order to avoid a substantial waste of material and provide uniformly thick and aesthetically appealing foam boards, it is highly desirable that the boards be produced with a substantially rectangular cross-section, and without a round top or other nonuniformity in the boards' dimensions. Heretofore a commonly practiced method for controlling the caliper or thickness of the boards has been the so-called "restrained rise" type. Here the foamable material and facers are directed between flatness-promoting confining means, such as a pair of spaced conveyor belts, which suitably are positioned apart a distance less than the thickness the combination of foam and facers would otherwise reach if allowed to expand unrestrictedly.
Another conventional method for continuously producing plastic foams is the "free rise" type, such as disclosed in U.S. Pat. No. 4,028,158, wherein the thickness of the finished foam board is controlled by passing the foamable mixture and facers through a metered gap which is preferably provided by the nip of two rotating rolls. Although this metering means avoids the time-consuming and cumbersome operation involved in using the conveyors of the restrained rise system, the caliper variation in the cross direction of foam boards produced on free rise lines, especially the thicker boards, turns out to be considerably greater than that of "restrained rise" boards. It would be highly desirable if a method could be found for producing foam boards having a high degree of uniformity in surface flatness combined with the other properties required in such boards without sacrificing the speed and efficiency which characterize free rise foam production lines.